db-jdo-dev mailing list archives

Hi Andy,
we have to distinguish optimistic and datastore transactions in this
discussion, and also
what we want to achieve.
Personally I think, we want to provide some behaviour guarantees of the
API. Unfortunately,
this is not the approach used by SQL for defining isolation levels.
So for datastore transactions it simply does not work, because one backend
might be a
versioning database while another is a non-versioning database, and the
behaviour will be
totally different, although both guarantee the same isolation level.
On the other hand with JDO optimistic transactions, the behaviour is quite
consistent right now (unless
flushing is involved), but only a two levels make sense:
READ_UNCOMMITTED
NO_LOST_UPDATES
all other levels are either unachievable or implicitly overachieved.
However, if we want to provide REPEATABLE_READ, then we could do so in
that we implicitly
include all read (but not modified) objects in the set of objects checked
for modifications at commit time.
Currently a user can do that, by calling "makeTransactional" on read
objects.
Best regards,
Christian
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From:
Andy Jefferson <andy@jpox.org>
To:
jdo-dev@db.apache.org
Cc:
JDO Expert Group <jdo-experts-ext@sun.com>
Date:
17.03.2008 10:21
Subject:
Re: JDO 2.2 : transaction isolation
Hi Christian,
Thx for your input.
> these levels are not really appropriate for JDO, which is because of the
> nature these are defined in the SQL spec:
> I. with a database guarantee of only committed reads and using JDO
> optimistic transactions, you effectively have an isolation level
stronger
> than TRANSACTION_READ_COMMITTED and lower than
> TRANSACTION_REPEATABLE_READ because the phenomenon of "unrepeatable
reads"
> can occur, while the phenomenon of "lost updates" can not occur.
> In our (old) implementation we thus introduced an additional isolation
> level "NO_LOST_UPDATES".
Ok.
> II. with a database guarantee of serializibility and using JDO
optimistic
> transactions, you don't achieve
> serializable JDO transactions unless the physical connection is bound to
> the PM for the whole JDO transaction duration.
Agreed.
There are 2 sides to this :-
1). Standardising a mechanism for specifying the transaction isolation
level.
This is what I am referring to, and to do that we need to provide a
notional
set of isolation levels - not necessarily just the JDBC set, but that was
the
start point as a basis for comment. As you say, that set is not complete
for
our scope, and other totally valid levels should be part of it. In some
parts
of the JDO interface (e.g value generation) we define some values, and
then
allow implementations to add on their own additional values if not catered
for in the defined list. This is what I would envisage. Is this realistic?
2). Standardising support for these levels in the JDO implementation, so
that
the user is always guaranteed to be able to use what they specify. I'm not
proposing this at all, and see that as unrealistic for an impl to provide
anyway. I simply propose that if an underlying datastore doesn't support
the
level specified then we throw an exception, hence the user always knows if
their isolation level is going to be used. This is very much in line with
other parts of the JDO spec where the implementation is free to support
some
or all of the valid values.
Obviously, where the underlying datastore supports multiple levels then it
provides value for the user. Similarly where the underlying datastore
supports only a single level then it is something that user would have no
need to change.
Regards
--
Andy (Java Persistent Objects - http://www.jpox.org)